T-Mobile USA's system is bog-standard IMS with RCS extensions. There are a few open source client stacks that support this, notably Doubango's. The trick is writing a frontend that uses regular Linux GUI libraries. As far as I know, nobody has done that yet...
Telepathy seems to be dying on desktop GNU/Linux, the existing clients in GNOME/etc are being replaced by protocol-specific applications. New protocol plugins seem to happen more in Pidgin/libpurple these days.
